1. Field of the Invention
The present invention relates to an optical mask and an exposure method using the optical mask, and more particularly, it relates to an optical mask for exposing resist to be used for the production of a semiconductor device, etc., and to exposure methods using the optical mask.
2. Description of the Related Art
In the development of VLSI's in recent years, it has become necessary to form patterns having dimensions of less than a half micron to realize high speed operation and high integration density. In order to realize such miniaturization, in the process of lithography, a basic method for pattern formation, it is indispensable to adopt an exposure system of reduction-type projection which is excellent in mass production and resolution.
However, the demand for miniaturization has outpaced the progress of devices. Thereby, the formation of a pattern size which surpasses the nominal resolution of a reduction-type projection exposure system is required, and it is necessary to draw out the performance of the system to the limit by the improvement in a resist material or a resist process. A phase shift exposure method is used in which a phase shift plate for reversing the phase of an exposure light is provided in a part of an optical mask as a technique to improve the limit of the performance of the system.
In the phase shift exposure method, an optical mask being disposed with transparent patterns of different phase shift angles is used. However, when the method is applied to an actual production of VLSI's, in some cases there is a difficulty in disposing the phase shifter patterns on a mask with consistency.
For example, when a pattern to be used for a semiconductor integrated circuit is separated into a plurality of parts and phase shifter patterns of 0.degree. and 180.degree. are disposed in respective parts, in some parts it can occur that a 0.degree. phase shifter pattern and a 180.degree. phase shifter pattern share a border with each other. A pattern which shifts the phase of a light by X.degree. is called an X.degree. phase shifter pattern.
In the above-mentioned contiguous part, the lights having different phase angles of 0.degree. and 180.degree. come in, so that the intensity of the light which passes through the contiguous part is remarkably lowered. Two methods can be considered to solve such a problem: one is to dispose a phase shifter pattern having a neutral phase angle, 90.degree., in the contiguous part of the 0.degree. phase shifter pattern and the 180.degree. phase shifter pattern; another one is to make a double exposure on the contiguous part using another optical mask. When the double exposure method is adopted, exposure is performed twice in changing optical masks, so that the throughput is decreased, and further, the alignment of masks is difficult and, thus, the through-put is lowered.
Therefore, an optical mask having a 90.degree. phase shifter pattern is generally adopted. The system is called a multistage system, and the system is proposed in the following papers, for example.
[1] SPIE vol.1463, 1991, pp101-110, H. Watanabe et al., "Transparent phase shifting mask with multistage phase shifter and comb-shaped shifter".
[2] SPIE vol.1463, 1991, pp327-335, J. Miyazaki et al., "A New phase-shifting Mask Structure for Positive Resist Process".
[3] SPIE vol.1463, 1991, pp197-206 "Variable Phase-Shift Mask for deep sub-micron optical lithography".
The examples of optical masks having 90.degree. phase shifter patterns are shown in FIGS. 1A, 2 and 3.
FIG. 1A shows a side of an optical mask for forming a pattern of lines and spaces. A plurality of transparent straight line patterns 101 are disposed having light shielding areas 102 in between. A 0.degree. phase shifter 103 and a 180.degree. phase shifter 104 are formed on each of the transparent patterns 101, and in the contiguous part of the 0.degree. phase shifter 103 and the 180.degree. phase shifter 104, a 90.degree. phase shifter 105 is disposed. The 0.degree. phase shifters 103 and 180.degree. phase shifters 104 are alternately disposed in the line width direction (up and down in FIG. 1).
In the case of this optical mask, 90.degree. phase shifters 105 are disposed in line in the direction of the line width, so that the lights which pass through the 90.degree. phase shifters 105 are diffracted to turn into the back of the light shielding areas 102 and the lights are superposed to strengthen each other. Further, the intensity of the light which is passed through the 90.degree. phase shifter 105 becomes small, as shown in FIG. 1B, influenced by the diffracted light. As a result, exposure margin is lowered and the probability of occurrence of a defect in a pattern is increased.
It is possible to dispose 90.degree. phase shifters 105 in zigzag in the direction of the pattern width (see FIG. 2). However, in that case, phase shifters 103, 104 and 105 having a phase difference of 90.degree. from each other are disposed in the direction of the pattern width with light shielding areas 102 in between, so that in comparison with the case of 180.degree. phase difference the effect of phase shifters is decreased. Thereby, the decrease in exposure margin cannot be avoided.
An optical mask shown in FIG. 3 comprises 2-comb shaped transparent patterns 106 being disposed as if the spine parts of the 2 combs are disposed facing each other, and the surroundings of the transparent patterns 106 are formed with light shielding areas 107.
In the spine parts, the (y) direction of the transparent patterns 106 having the shape of 2 combs, there are formed 0.degree. phase shifters 108 and 180.degree. phase shifters 109 alternately disposed along the (y) direction and being extended in the (x) direction. Further, among the line patterns in the (y) direction corresponding to the spine parts, in the part which is contiguous with 0.degree. phase shifters 108, there are disposed 90.degree. phase shifters 110, and in the other parts, 180.degree. phase shifters 109 are disposed and extend to the 90.degree. phase shifters 110.
In the case of this optical mask, on the back of a light shielding portion 111, which is disposed between the transparent patterns 106 having the shape of 2 combs, the lights being passed through the phase shifters 108, 109 and 110, disposed on both sides of the light shielding portion 111 and having the phase difference of 90.degree. from each other, are diffracted and superimposed on each other, so that the exposure margin is lowered.